Ocean waves in moderate to high sea states present high power densities and consequently represent an attractive energy source for power generation using renewable resources. Much work on wave energy converters (WEC's) has been done and many proposals for exploiting ocean-wave power have been documented.
An early approach to wave energy extraction was based on a single-degree-of-freedom system, for which an oscillating body was directly coupled to a power take-off mechanism that was suspended on the bottom. Heave-excited WEC's with a single degree of freedom generate relatively low velocity motions that are not high enough to use practical direct-drive electrical generators unless mechanical transformers are engaged. Moreover, it may be difficult to provide suitable connection hardware from the buoy to the reference platform on the sea bottom due to tidal action and the very high forces involved. Systems with two degrees of freedom overcome bottom connection problems, a necessity in deep water.